Polyimides have physical properties that make them especially suited for microelectronic applications. These properties include high thermal stability, low dielectric constant, excellent adhesion to substrates, high radiation stability, excellent planarization, and high flexibility relative to inorganic dielectrics. Thus, polyimide layers are widely used in the electronics industry to provide protection, electrical insulation, or both. They are used, for example, as protective coatings for semiconductors, as dielectric layers for multilayer integrated circuits, as high-temperature solder masks, as bonding for multilayer circuits, and as the final passivation coating on electronic devices.
Polyimide layers may be formed by vapor deposition techniques in which a precursor is deposited on the substrate and then thermally cured. Vapor deposition techniques are slow and impractical, however, when thick layers (greater than 500 nm) are desired. Alternatively, polyimide layers may be formed by cross-linking solution-deposited polyimide precursor. The total thickness that can practically attained in a single layer is limited, however, to about 25 microns. If a thicker layer is desired, it is necessary to apply a second layer of polyimide precursor on top of the first polyimide layer. In addition, in some applications, it is necessary to form a second polyimide layer, or a layer of a different polymer, on a previously applied polyimide layer to provide protection or insulation.
Poor adhesion results at the interface between the first polyimide layer and the second layer of polymer. To improve the adhesion between the layers, it is necessary to treat the surface of the first layer before the second layer is deposited. Treatment typically requires wet chemical processing. One such method, for example, involves the hydrolysis of cured polyimide film with an alkaline solution in a solvent or a mixture of solvents to produce a polyamic acid salt, which is transformed to a polyamic acid-containing surface onto which a liquid polyimide precursor is deposited and subsequently cured. See, for example, L. J. Matienzo and W. N. Unertl, “Adhesion of Metal Films To Polyimide,” in Polyimides: Fundamentals and Applications, Chapter 21, p. 659 (M. K. Gosh & K. L. Mittal, eds., Marcel-Dekker, New York, 1996); and N. C. Stoffel, “Interdiffusion and Adhesion of Polyimides,” Ph.D. Thesis, Cornell University (1995).
Thus, a need exists for a method for improving the adhesion between a polyimide layer and a second layer of polymer, especially a second polyimide layer, that does not require wet chemical processing.